Palaeogeography, Palaeoclimatology, Palaeoecology Elsevier Publishing Company, Amsterdam - Printed in The Netherlands PALEOMAGNETISM OF SOME GONDWANA RED BEDS FROM CENTRAL INDIA H. WENSINK Geological Institute, State University, Utrecht (The Netherlands') (Received July 2, 1968) (Resubmitted October 30, 1968) SUMMARY Oriented samples were collected for paleomagnetic research from red horizons of the Gondwana sediments at a few localities in central India. The specimens were subjected to both alternating-field and thermal demagnetization and were measured on astatic magnetometers. Usually a stable direction of magnetization was observed only after thermal treatment. The following results were obtained: (1) The Upper Permian Kamthi Beds reveal a mean direction of magneti- zation given by D = 79.4 °, I = +61.6 ° (pole position 21.05°S 50.4°W; dp= 2.8 ° and dm = 3.7°); (2) The Lower Triassic Mangli Beds have D = 100.1 °, I = +62.8 ° (pole position 7.3°S 55.7°W; dp = 5.7 ° and dm = 7.3°); (3) The Upper Triassic Pachmarhi Beds have D = 117.2 °, I = +48.9 ° (pole position 10.15°N 49.9°W; dp = 4.0 ° and dm= 6.1 °). These results combined with earlier data indicate that the Indian sub- continent has made a sinistral rotation of about 90 ° during Upper Paleozoic through Mesozoic times, without a considerable meridional shift. INTRODUCTION The present study deals with the paleomagnetism of rock samples from the Indian Gondwana sediments, collected during a field trip in the winter season of 1966-1967. The Indian subcontinent is considered to have formed part of the Gondwana continents of the Southern Hemisphere, because of paleontological similarities. Paleomagnetic studies on the Upper Cretaceous to Lower Eocene plateau basalts of the Deccan traps (DEuTSCH et al., 1959) as well as on the Jurassic Rajmahal traps (CLEGG et al., 1958) and Sylhet traps (ATHAVALE et al., 1963) have demonstrated that during the Upper Mesozoic India was situated south of the equator. Palaeogeography, Palaeoelimatol., Palaeoecol., 5 (1968) 323-343 324 H. WENSINK The paleomagnetic research is now extended to the, mainly Mesozoic, sediments of the Gondwana System (VERMA et al., 1967, 1968). This paper presents some additional paleomagnetic data from the Gondwana sediments. GEOLOGICALASPECTS OF THE INDIAN PENINSULA The Indian peninsula consists of a basement of Precambrian metamorphic rocks and associated granitic intrusions belonging to the Dharwar System. Locally they are overlain by Late Precambrian sediments and volcanics of the Cuddapah and Vindhyan Systems. Rocks of a Lower Paleozoic age are not found on the Indian subcontinent. The next stratigraphical unit is formed by a continental series which starts with glacial boulder beds and their associated peri-glacial beds of the Late Car- boniferous glaciation. From that time onwards sedimentation continues through the greater part of the Mesozoic until the Lower Cretaceous. All these deposits are classified in the Gondwana System. The rocks of the Gondwana System, which are chiefly composed of sand- stones and shales, are almost entirely deposited in fresh water. Intercalations of coal measures occur; they are mainly found in the lower part of the system. The subdivision of the Gondwana System into two subsystems is based on the content of plants: the Lower Gondwana System is characterized by the Glossopteris flora; the younger Upper Gondwana System can be distinguished from it by a cycadaceous Ptilophyllum flora. The transition between the Lower and the Upper Gondwana Systems takes place in the Upper Triassic. Apart from the subdivision into two separate systems advocated both by PASCOE (1959) and by KRISHNAN (1960), another subdivision is given by WADIA (1953) viz., into three systems. Wadia distinguishes the Lower, the Middle, and the Upper Gondwana Systems. These coincide with the Upper Carboniferous-Permian, the Triassic, and the Jurassic together with the Lower Cretaceous respectively. The Gondwana sediments are deposited in a number of basins. The local stratigraphies which have been drawn up for the individual basins do not facilitate the establishment of a stratigraphy that is representative for the whole Gondwana System. The preservation of the Gondwana sediments is largely a result of faulting subsequent to the deposition of the beds concerned. A vast area of the western and central parts of the Indian peninsula is covered by a series of plateau basalts-the Deccan traps-which have a thickness of at least 1,500 m. The lavas were extruded during the Upper Cretaceous-Lower Eocene. The northern part of the subcontinent of India is covered by Quaternary sediments of the Ganges and Indus plains. Palaeogeography, Palaeociimatol., Palaeoecol., 5 (1968) 323-343 PALEOMAGNETISM OF GONDWANA RED BEDS FROM CENTRAL INDIA 325 SAMPLING IN THE FIELD Locally the sediments of the Gondwana Systems contain intercalations of reddish clays, shales and sandstones. From these red sediments, which may be suitable for a paleomagnetic research, oriented samples were collected at a few localities in the central Indian peninsula. The sampling in the field was carried out with a portable diamond drill provided with a coring tube of 2.5 cm inner diameter. However, sometimes the Gondwana sediments are almost unconsolidated, and this prevents successful drilling; in that case oriented handsamples were taken. Samples from the Upper Carboniferous Talchir Beds were taken on two localities (I and II in Fig.l). Only the laminated, unconsolidated red clays near Nagpur (locality I in Fig.l; site ITLA in Fig.2) did reveal a stable direction of magnetization (WENSINKand KLOOTWIJK,1968). 76 78 80E -- o o o 2 ÷-: .E CENT , __ + DECCAN TRAPS SEDIMENTS ~' Ir-~" ~ UPPER PRE'C ~ it,D 200/~~ _~-+-F'FT~ SEDIMENTS BASEMENT 78 80 Fig. 1. Geological map of central India with the sampling localities; locality I corresponds to site ITL-A; H to ITL-B; 11I to IWU-A, -B, -C, -D, -E, and -H; 1V to IML-A, and -B; l/to ITA-A, and -B; F1 to IPC-B, -C, and -D. In the northwestern part of the Godavary basin (localities III, IV, and V in Fig.l) sediments of the Lower Gondwana System are exposed in an almost flat area. Near Wun (locality III in Fig. 1) outcrops of grey sandstones are found, which contain intercalations of red shales. The sediments near Wun are generally assigned to the Upper Permian Kamthi Beds (PASCOE, 1959). In six IWU (India Palaeogeography, Palaeoclimatol., PalaeoecoL. 5 (1968) 323-343 326 H. WENSINK 3LASSIFICATI ON STAGE, c SITES AGErn DECCAN TERTIARY I SEVERAL TRAPS -64 - CRETAC- EOUS c -u m i- 136- JURASSIC PACHMARHI IPC-B- D -190- -r--- TRIASSIC o m MANGLI I M L-A-B -225- KAMTHI I W U-A-H 6") PERMIAN o I T A-A- B z UPPER CARB .~ TALCHIR IT L-A- B Fig.2. Schematic stratigraphical table of the Gondwana System from India indicating the Stages from which samples were collected for paleomagnetic research. WUn) sites: IWU-A, -B, -C, -D, -E, and-H, cores were drilled in these shales at three different places. At each site 12-15 cores were taken. In the stratigraphical sequence the sites IWU-E and -H are situated about 70 m lower than the sites IWU-C 1 and -D. The stratigraphical position of the remaining sites IWU-A and -B is uncertain compared with that of the former four sites (see also p. 338). Another two sites (ITA-A and -B; V in Fig.l) are located at Tadoba north of Chanda. In a series of sandstones and conglomerates an intercalation of red, sandy shales occurs, from which oriented cores were taken. The sites ITA-A and -B, furnished about fifteen cores each. These sediments may belong to the Kamthi Beds. Near the village of Mangli, south of Nagpur, i.e., the type locality of the Lower Triassic Mangli Beds, oriented cores were drilled from two sites (IML-A and -B; IV in Fig.l) in red shales intercalated in white sandstones. At each of the sites IML-A and -B, fourteen cores were collected. Pebbles of these red shales were found within a conglomeratic layer that occurs later in the succession. Only at the outside are the shale pebbles somewhat decolourized. The presence of these pebbles in overlying conglomerates proves that the colouration of the shales is (almost) synsedimentary. 1 Reliable paleomagnetic results of this site have not yet been obtained. Palaeogeography, Palaeoclimatol., Palaeoecol., 5 (1968) 323 343 PALEOMAGNETISM OF GONDWANA RED BEDS FROM CENTRAL INDIA 327 The Gondwana sequence in the Satpura area consists of a slightly north- ward dipping series and contains both the Lower and a part of the Upper Gond- wana System. Near Pachmarhi coarse sandstones and conglomerates are met with, which belong to the Upper Triassic-Lower Jurassic Pachmarhi Beds. South of the village oriented cores were collected from three sites (1PC-B, -C, and -D; VI in Fig. l) in reddish fine-grained sand- and siltstones which are intercalated in this succession. These three sites furnished about fifteen cores each. ANALYSIS OF THE MAGNETIZATION General The samples collected in the field were investigated at the Paleomagnetic Laboratory of Utrecht University. The handsamples were embedded in their oriented positions in cubes of paraffine with sides of 10 cm. The cores were sawn into cylinders of 22 mm length. From some of the cores more than one cylinder could be obtained. In order to obtain the intensity and the direction of the Natural Remanent Magnetization (N.R,M.) and its analysis all samples were measured on astatic magnetometers. Demagnetization in alternating magnetic fields (a.c.) The change of the total magnetization vector during the demagnetization procedure can best be illustrated in orthogonal projections in a rectangular axis system. This has the advantage that exact information can immediately be obtained about the change in both the intensity and the direction of magnetization of the specimen involved.